The influence of a reduced plantar support surface area on the compensatory reactions to a forward fall.

The influence of altered plantar support area on the compensatory reactions to forward fall was studied. The forward fall was provoked from an initial forward inclined posture, and balance was recovered by making steps. Alteration of the support area was unilateral and was obtained using two rods on which the subject stood. The rods were under either the starting foot or the stance foot. The configuration of the plantar support area, flat or reduced, did not change the latency of Soleus electromyographic activity. In contrast, the unilateral reduction of the plantar support area had powerful bilateral effects on the magnitude of Soleus activity. The reduction of the plantar support area on the swing side significantly reduced the ipsilateral Soleus response. The reduction of plantar support area on the stance side did not alter the initial part of the ipsilateral soleus response, but did change the late response (corresponding to the unipedal stance). Reduction of plantar support surface area resulted in earlier step execution and earlier heel-contact, i.e. faster balance recovery. These changes suggest that there is an integration of the spinal reflexes, as well as the stepping motor program.